Explanation:

- Calcium: While calcium is crucial for plant structure and signaling, it isn’t directly used in protein synthesis.

- Sodium: Sodium isn't essential for most plants. It's not involved in their protein synthesis.

- Chlorine: Plays a role in photosynthesis but not in making proteins.

- Nitrogen: Essential for protein synthesis. It's a key part of amino acids, the building blocks of proteins, and is vital for plant growth and development.